In the endoplasmic reticulum (ER), ATP is essential for the folding, maturation, transport and degradation of proteins. As there is no pathway for endogenous ATP synthesis in the ER, these constant processes all rely on ATP import from the cytosol, but it is not until recently that the responsible transporter has been identified.

ATP is a universal energy currency for all cells, ranging from bacteria to humans. In eukaryotic cells, ATP is mainly synthesized in mitochondria and exported to the cytosol, from where it is imported into other compartments to serve as fuel for multiple biological processes. In fact, mitochondrial carriers of the SLC25 family move our own body weight in ATP every day (Ruprecht, 2020).

SLC35B1 is a member of the nucleotide sugar transporter (NST) family, but a recent publication in Nature demonstrated that the human version of this transporter actually executes ATP/ADP exchange, as determined by both genetic, structural and biochemical analyses (Gulati, 2025). This is in agreement with a role as an ER-resident ubiquitous carrier that catalyzes the uptake of ATP, as already proposed in 2018 (Klein, 2018). As noted by Klein et al., SLC35B1 (also known as AXER) localizes to the ER in all three cell lines stained by ICC-IF in the subcellular section of the HPA.

Ulrika Axelsson